<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function runyear(year) {
        if ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0) {
          return true;
        } else {
          return false;
        }
      }
      function add(year) {
        var a1 = runyear(year);
        
        if (a1) {
          alert('2月是29天');
        } else {
          alert('2月是28天');
        }
      }
     
      add(2001);
      //   console.log(result);
      //扩展
      // var add = prompt('输入年份');
    </script>
  </body>
</html>
